Someone more technical can probably help you more, but here's what I see with my car (has a M104 CIS-E engine):

Cold
1000-1100 in P or N
700-800 in D or R

Warm (80deg)
700-800 in P or N
500-600 in D or R

From what I've read on different forums this is all normal behaviour.

Did have a problem with my micro switch (think full name is: "Deceleration throttle micro-switch") a few months ago when the garage managed to adjust my throttle linkage such that the switch was no longer activated at idle (i.e. my foot completely off the accelerator). What I saw was the cold idling uneffected, but idling when warm became 1400-1500 and engine temp at idle shot up over 100deg.

All I had to do was turn a little black plastic adjustment screw on the linkage a half turn, which slackened off the cable sufficiently for the spring to pull the arm more forcefully onto the switch. Immediately the switch clicked the idle revs dropped to 500-600!

Hope that's of some help. Also see the thread "300SE high/sticking idle" which has some more info.